Thermophilic Composting of Human Feces: Development of Bacterial Community Composition and Antimicrobial Resistance Gene Pool

Biochar preparation and evaluation of its effect in composting mechanism: A review

This article provides an overview of biochar application for organic waste co-composting and its biochemical transformation mechanism. As a composting amendment, biochar work in the adsorption of nutrients, the retention of oxygen and water, and the promotion of electron transfer. These functions serve the micro-organisms (physical support of niche) and determine changes in community structure beyond the succession of composing primary microorganisms. Biochar mediates resistance genes, mobile gene elements, and biochemical metabolic activities of organic matter degrading. The participation of biochar enriched the α-diversity of microbial communities at all stages of composting, and ultimately reflects the high γ-diversity. Finally, easy and convincing biochar preparation methods and characteristic need to be explored, in turn, the mechanism of biochar on composting microbes at the microscopic level can be studied in depth.

Composition, structure, and functional shifts of prokaryotic communities in response to co-composting of various nitrogenous green feedstocks

BACKGROUND

Thermophilic composting is a promising method of sanitizing pathogens in manure and a source of agriculturally important thermostable enzymes and microorganisms from organic wastes. Despite the extensive studies on compost prokaryotes, shifts in microbial profiles under the influence of various green materials and composting days are still not well understood, considering the complexity of the green material sources. Here, the effect of regimens of green composting material on the diversity, abundance, and metabolic capacity of prokaryotic communities in a thermophilic compost environment was examined.

METHODS

Total community 16S rRNA was recovered from triplicate compost samples of Lantana-based, Tithonia-based, Grass-based, and mixed (Lantana + Tithonia + Grass)- based at 21, 42, 63, and 84 days of composting. The 16S rRNA was sequenced using the Illumina Miseq platform. Bioinformatics analysis was done using Divisive Amplicon Denoising Algorithm version 2 (DADA2) R version 4.1 and Phylogenetic Investigation of Communities by Reconstruction of Unobserved States version 2 (PICRUSt2) pipelines for community structure and metabolic profiles, respectively. In DADA2, prokaryotic classification was done using the Refseq-ribosomal database project (RDP) and SILVA version 138 databases.

RESULTS

Our results showed apparent differences in prokaryotic community structure for total diversity and abundance within the four compost regimens and composting days. The study showed that the most prevalent phyla during composting included Acidobacteriota, Actinobacteriota, Bacteroidota, Chloroflexi, and Proteobacteria. Additionally, there were differences in the overall diversity of metabolic pathways but no significant differences among the various compost treatments on major metabolic pathways like carbohydrate biosynthesis, carbohydrate degradation, and nitrogen biosynthesis.

CONCLUSION

Various sources of green material affect the succession of compost nutrients and prokaryotic communities. The similarity of amounts of nutrients, such as total Nitrogen, at the end of the composting process, despite differences in feedstock material, indicates a significant influence of composting days on the stability of nutrients during composting.

Fate of Horizontal-Gene-Transfer Markers and Beta-Lactamase Genes during Thermophilic Composting of Human Excreta

Thermophilic composting is a suitable treatment for the recycling of organic wastes for agriculture. However, using human excreta as feedstock for composting raises concerns about antibiotic resistances. We analyzed samples from the start and end of a thermophilic composting trial of human excreta, together with green cuttings and straw, with and without biochar. Beta-lactamase genes bla_CTX-M, bla_IMP, and bla_TEM conferring resistance to broad-spectrum beta-lactam antibiotics, as well as horizontal gene transfer marker genes, intI1 and korB, were quantified using qPCR. We found low concentrations of the beta-lactamase genes in all samples, with non-significant mean decreases in bla_CTX-M and bla_TEM copy numbers and a mean increase in bla_IMP copy numbers. The decrease in both intI1 and korB genes from start to end of composting indicated that thermophilic composting can decrease the horizontal spread of resistance genes. Thus, thermophilic composting can be a suitable treatment for the recycling of human excreta.

Effects of Combined Application of Compost and Mineral Fertilizer on Soil Carbon and Nutrient Content, Yield, and Agronomic Nitrogen Use Efficiency in Maize-Potato Cropping Systems in Southern Ethiopia

Low nutrient input and low soil fertility are limiting agricultural productivity in Ethiopia. The main objectives were therefore to evaluate the effects of combined compost and mineral fertilizer (MF) application on soil properties, yield, agronomic nitrogen use efficiency, and adoption of compost application in maize (Zey mays L.)—potato (Solanum tuberosum L.) cropping systems. Yield data were collected from 20 smallholders applying (i) compost and MF at a total rate of 110 kg N ha−1, with 6 Mg compost ha−1 + MF (6CF), 12 Mg compost ha−1 + MF (12CF), and 16 Mg compost ha−1 + MF (16CF; compost on a fresh weight basis), (ii) MF application of 108 kg N ha−1 (F), and (iii) zero fertilization. Soil from 0–20 and 20–40 cm depths was collected from 16 farms using compost and MF. Compost + MF treatments showed significantly lower soil bulk density and iron contents, while pH, electrical conductivity, and cation exchange capacity were higher compared to F treatments. The 6CF, 12CF, and 16CF showed 22, 43, and 54% higher maize grain yield and 8, 16, and 18% higher potato tuber yield compared to F, respectively. The scarcity of organic material was a major socioeconomic constraint for smallholders for producing and applying compost.

Metagenomic Insights Into the Changes of Antibiotic Resistance and Pathogenicity Factor Pools Upon Thermophilic Composting of Human Excreta

In times of climate change, practicing a form of sustainable, climate-resilient and productive agriculture is of primordial importance. Compost could be one form of sustainable fertilizer, which is increasing humus, water holding capacity, and nutrient contents of soils. It could thereby strengthen agriculture toward the adverse effects of climate change, especially when additionally combined with biochar. To get access to sufficient amounts of suitable materials for composting, resources, which are currently treated as waste, such as human excreta, could be a promising option. However, the safety of the produced compost regarding human pathogens, pharmaceuticals (like antibiotics) and related resistance genes must be considered. In this context, we have investigated the effect of 140- and 154-days of thermophilic composting on the hygienization of human excreta and saw dust from dry toilets together with straw and green cuttings with and without addition of biochar. Compost samples were taken at the beginning and end of the composting process and metagenomic analysis was conducted to assess the fate of antibiotic resistance genes (ARGs) and pathogenicity factors of the microbial community over composting. Potential ARGs conferring resistance to major classes of antibiotics, such as beta-lactam antibiotics, vancomycin, the MLS_B group, aminoglycosides, tetracyclines and quinolones were detected in all samples. However, relative abundance of ARGs decreased from the beginning to the end of composting. This trend was also found for genes encoding type III, type IV, and type VI secretion systems, that are involved in pathogenicity, protein effector transport into eukaryotic cells and horizontal gene transfer between bacteria, respectively. The results suggest that the occurrence of potentially pathogenic microorganisms harboring ARGs declines during thermophilic composting. Nevertheless, ARG levels did not decline below the detection limit of quantitative PCR (qPCR). Thresholds for the usage of compost regarding acceptable resistance gene levels are yet to be evaluated and defined.